Sampling clock offset, including sampling phase offset and sampling frequency offset, commonly exists in both wired and wireless communication systems. Sampling phase offset refers to the offset that the actual sampling instant deviates from the optimal sampling instant, while sampling frequency offset refers to the sampling drift that caused by the inconsistency between the crystal oscillator frequency at the sending end and the crystal oscillator frequency at the receiving end in a communication system. In a digital system, sampling frequency offset represents as an increase or decrease of the number of sampling points within the same time span in the time domain, and such increase or decrease of sampling points will accumulate over time. Sampling phase recovery and sampling frequency recovery are collectively referred to as sampling recovery.
For an OFDM receiver, if there only exists sampling phase offset, it will only result in a phase rotation of each sub-carrier signal, which will not have a significant impact on the signal-to-noise ratio at the receiving end of the system after carrying out a channel compensation. In contrast, if there exists sampling frequency offset, on one hand, it will result in the start time drift of the FFT (Fast Fourier Transform) window; and on the other hand, will destroy the orthogonality between subcarriers, resulting in inter-carrier interference and causing the reduction of the signal-to-noise ratio of the system. For a spread spectrum communication system, either sampling phase offset or sampling frequency offset will result in an inaccurate optimal matching time and therefore affect the correlation peak after the matching. More seriously, sampling frequency offset will lead to continuous drifting of the optimal sampling instant, as a result, the signal cannot be demodulated normally.
Early-late gate is the most fundamental and the most important algorithm principle in sampling recovery. Early-late gate algorithm extracts sampling phase offset information by detecting an optimal instant sampling point as well as the sampling points before and after it, and adjusts the sampling frequency offset by continuously tracking changes in the sampling phase offset. The early-late gate is generally employed at a high rate to extract sampling phase offset information in a spread spectrum communication system. This requires the system to operate at a high rate, and therefore increases the implementation complexity of the system.
Generally, an OFDM system obtains sampling phase offset information through conjugate multiplication of the pilots located at the same subcarrier position of two consecutive OFDM symbols. As this algorithm is based on the assumption that the channel responses of the two consecutive OFDM symbols remain substantially unchanged, it fails to have good performance in a mobile environment. Moreover, in order to maintain a satisfying transmission efficiency, the number of pilots is typically very limited, which will largely restrain the performance of the algorithm. ‘Timing Recovery for OFDM Transmission’ published in IEEE in 2000 proposed a sampling recovery method using locally generated lead and lag pilots. This method essentially belongs to the principle of early-late gate algorithm. However, the method has two shortcomings as follows: 1) as it performs estimation by using pilot subcarriers, its performance is affected by the number of pilots; 2) it needs a reference path for a phaselocked loop to lock the sampling phase and the sampling frequency eventually, but as any path may undergo fading and even elimination in the mobile environment, locking failure will occur in the algorithm during mobile reception.
CN102075475A and CN101534184A disclose sampling recovery methods for CMMB and DTMB systems, respectively. The methods are based on a fundamental concept to continuously correct sampling frequency offset by tracking the drift of the strongest path at a symbol rate. These methods have two defects as follows: 1) the algorithm itself is designed only for sampling frequency offset correction, not for sampling phase offset correction; 2) it can be affected by mobile environment and multipath distribution since the adjustment interval is very long, and therefore, it may be impossible to extract timing information in a mobile environment having a continuously changing multipath or a multipath with very complicated distribution.
CN1677910A discloses a sampling recovery method for DTMB system, and CN101645861A discloses a method for extracting sampling offset. Both of the methods extract sampling offset information from the strongest path in a transmission channel, and then carry out sampling recovery by using a phaselocked loop. Although these methods have better performance in multipath channels and mobile channels, they still have problems as follows: 1) due to the delay effect in the operation of a phaselocked loop, it always take some time for the phaselocked loop to reach a stable state, which is disadvantage for burst communication; 2) when the multipath changes dramatically, the algorithm needs to track back and forth to lock a different path, which will cause the phaselocked loop to keep switching between the capture mode and the track mode, and is significantly disadvantage for the stability of the algorithm; and 3) in order to ensure the precision of sampling recovery, the system is required to operate at a high sampling rate which is much higher than the rate needed for data demodulating operation, thus increasing the implementation complexity of the system and wasting the hardware resources and processing time.